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close this bookAppropriate Community Technology - A Training Manual (Peace Corps; 1982; 685 pages)
View the documentThe Farallones Institute Rural Center
View the documentCHP International, INC.
View the documentPreface
View the documentAcknowledgments
View the documentIntroduction
Open this folder and view contentsPhase I: Introduction to training
Open this folder and view contentsPhase II: Earthen construction and fuel-saving cookstoves
Open this folder and view contentsPhase III: Pedal/treadle power
Open this folder and view contentsPhase IV: Solar water heaters
close this folderPhase V: Solar agricultural dryers
View the documentPhase V Calendar
View the documentSession 1. Introduction to agricultural dryers
View the documentSession 2. Tour of solar dryers
View the documentSession 3. Solar agricultural dryer design procedures and rules of thumb
View the documentSession 4. Two-hour dryer construction
View the documentSession 5. Review of existing solar dryer plans
View the documentSession 6. Smoke testing solar dryers
View the documentSession 7. Introducing new technologies: solar dryers
View the documentSession 8. Design of solar agricultural dryers
View the documentSession 9. Site selection and preparation
View the documentSession 10. Construction of solar agricultural dryers
View the documentSession 11. Issues and methods in development and diffusion of appropriate technology
View the documentSession 12. Natural cooling
View the documentSession 13. Approaches to health systems
View the documentSession 14. Nutritional gardening
View the documentSession 15. Practical drying tips
View the documentSession 16. Cardiopulmonary resuscitation
View the documentSession 17. Dryer assessment and modification
View the documentSession 18. Introduction to cost benefit analysis (cba)
View the documentSession 19. Presentation of solar dryers
View the documentSession 20. Introduction to the final phase of the training program
Open this folder and view contentsPhase VI: Concluding the program: The energy fair
Open this folder and view contentsAppendices

Session 15. Practical drying tips

Total time:

2 hours


* To taste and examine rehydrated foods


* To discuss practical solar food drying techniques and guidelines


* To discuss the effect of drying on the nutritive value of foods


* Attachment V-15-A, "Summary of Temperature Factors that Affect the Preservation and Drying of Foods"


* Attachment V-15-B, "Practical Food Dehydration"


Various solar-dried foods (fruits, vegetables, fish, meat, etc.) stored in appropriate containers

Trainer Notes

It is essential to this session that the trainer have extensive practical experience in solar food drying, storing and preparation. If necessary, invite a local consultant with these skills to conduct this session.

You will need to rehydrate some solar-dried foods for tasting during this session. This can be done quickly by soaking them overnight or by boiling them.


Step 1. (15 minutes)
Have the participants sample some of the solardried foods which have been rehydrated.

Trainer Notes

Encourage discussion of the texture, appearance, flavor, color, odor, etc. of the foods.

Step 2. (5 minutes)
Explain the session objectives and outline the activities.

Step 3. (30 minutes)
Give a short talk on practical food drying techniques, methods and guidelines. Encourage questions and discussion.

Trainer Notes

Your talk should include the following topics:

* Advantages and reasons for drying foods
* Insect/pest problems and solutions
* Storage techniques
* Recipes
* Time-saving techniques
* Avoiding drying too much or too little

Step 4. (25 minutes)
Distribute and discuss Attachments V-15-A and V-15-B, answering questions and encouraging discussion.

Step 5. (30 minutes)
Discuss the nutritive aspects of drying foods.

Trainer Notes

* Indicate those vitamins which are preserved and those which are lost in the drying process.

* Stress Vitamin C as being particularly fragile, subject to degradation by light and heat and best obtained from fresh foods.

Step 6. (15 minutes)
Conclude by reviewing the objectives and answering final questions.



Blanch (full steam) at 100°C, Boil water to sterilize for treatment solutions for 20 minutes.

Pasteurize fruit for 15 minutes and vegetables for ten minutes at 80°C.

Over 60°C, food will cook, scorch and lose nutrients.

Food kept in dryer at 57°C for one hour is pasteurized sufficiently.

The range of 45°C to 60°C is good for drying food quickly with little loss of nutrients and color and protection from microorganisms and enzyme action.

Fish will cook in direct sunlight even in the 20°C to 30°C range.

Below 40°C in a humid climate, food can spoil on the drying trays.

Store dried meat and fish at 5°C or below. High protein foods spoil more easily than other foods.


Operation of the dehydrator is not complex but requires conscientious, systematic attention. Each operator should develop a satisfactory method of food dehydrating to match his/her climate, daily schedule and food source. The following guidelines will help to establish a dehydration system.

1. Choose fresh, ripe (not overripe), undamaged foods.

2. Slice food into long, thin slices, less than 1/2" thick. Cut meat or fish into small chunks for soups or stews. Use a stainless steel knife to minimize discoloration. Cut and prepare foods quickly. Hake all pieces uniform in size.

3. Lay pieces close to each other without over Tapping. Allow for air to circulate between pieces.


2.5 lbs. per square foot of tray with pre-heater


1.5 lbs. per square foot of tray without pre-heater

4. Blanching is not advised but will aid preservation in some cases.

5. Rotate food trays 180° once each day for even drying.

6. Weigh food before drying and after three days of drying. (Weigh without trays for better accuracy.) Food is "dry for storage" if final weight is 1/5th of original. (This varies depending on percentage of moisture in various foods.) Remove 88%, of moisture to stop spoilage. (See formula below.)

7. Dehydration time depends on humidity, type of food, food moisture, percent sunshine, etc. Three to four sunny days is average.

8. Use a thermometer in the cabinet to maintain temperature by manipulating vent damper.

Dry Weight Formula

Dry Weight

= (original weight of food) - (original weight percent moisture) + (.10 x original weight x % moisture)

Dry Weight

= (original weight) x (1 -.9 [Moisture of food])

Example: What will be the dry weight of 6 lbs. of apricots if their original moisture is 92%?

Dry Weight

= 6 lbs. x (1 - .9 [.92])


= 6 lbs. x (.17)


= 1.02 lbs. (Therefore, dry the apricots until they weigh one lb.)

Storing Dehydrated Foods

1. Store in a cool, dark, dry place, in small, airtight containers.

2. Store in paper bag or cheesecloth for one week to absorb surface moisture. Transfer to glass jars or hard plastic containers.

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